1. Field of the Invention
The present invention relates to a carbon thin film made of tetrahedral amorphous carbon, which is for protecting a base material, a mold for molding an optical element having the carbon thin film on the surface, and a method for manufacturing an optical element in which the mold for molding an optical element is used.
2. Description of Related Art
A carbon thin film made of tetrahedral amorphous carbon (hereinafter, sometimes abbreviated to a ta-C thin film) is put into practical use as a film that protects the base material of a mold, a drill, and the like.
In addition, as a method for forming the ta-C thin film, an ion plating method, including arc ion plating, in which carbon ions are deposited on the base material using a bias voltage applied to the base material, is mainly applied. According to this film-forming method, since many of sp3 bonding (sp3 bonds), which are bonds between carbon atoms that form a diamond, can be included as bonds between carbon atoms in the ta-C thin film, it is possible to make the ta-C thin film into an extremely hard (high hardness) film.
In the ta-C thin film including sp3 bonds, carbon atoms form a strong network of the sp3 bonds in a small area, but, in view of a large area, an order less amorphous structure is formed, and therefore the surface becomes extremely flat and smooth compared to a thin film made of a crystal having a wholly-ordered structure, and the coefficient of friction becomes as extremely small as 0.1 or less. As such, since the ta-C thin film can form a flat and smooth film having an extremely high hardness on a base material, the ta-C thin film is preferable as a protective film for the portions of a mold, a drill, or the like which slide on work pieces, and is widely used.
However, since a high-hardness ta-C thin film has an extremely large internal stress (compressive stress), there is a problem in that, when a shock (contact shock) or a thermal shock is applied from the outside, its own internal stress is released, and the ta-C thin film becomes liable to be peeled off from the base material. In addition, when an excessively thick film is formed, the internal stress further increases, and therefore the ta-C film becomes more liable to be peeled off. Therefore, it is not possible to employ a method for simply increasing the film thickness for the purpose of improving the durability, which is an extension of a period of time during which the thin film is abraded due to sliding on work pieces so as to be removed on the base material. In order to suppress the peeling of the ta-C thin film from the base material, it is considered to decrease the amount of the sp3 bonding in the ta-C thin film so as to decrease the internal stress, and to increase the adhesion with the base material. However, according to this method, the hardness of the thin film also decreases at the same time, and therefore it is not possible to improve abrasion resistance. As such, it is extremely difficult to improve abrasion resistance in a ta-C thin film having a uniform amount of the sp3 bonding.
In the present specification, the “amount of the sp3 bonding” refers to the percentage of the number of the sp3 bonding in the total number of sp2 bonding and the sp3 bonding in the ta-C thin film.
Thus far, as a method for manufacturing a ta-C thin film, a method has been disclosed in which, when the ta-C thin film is formed on a base material, mixing between the base material and the thin film is strengthened by increasing a bias voltage, furthermore, another thin film having a large amount of sp3 bonding and a high hardness is formed on the thin film so as to produce a bilayer-structured ta-C thin film, and thus, adhesion with the base material is improved (refer to Japanese Unexamined Patent Application, First Publication No. 2007-169698). In addition, a method is disclosed in which a ta-C thin film structured by alternately laminating two layers having different amounts of sp3 bonding is produced, and abrasion resistance is improved (refer to Japanese Unexamined Patent Application, First Publication No. 2008-1951).